There is already known a method of analyzing whether there is a specific type of object gas of analysis, gas concentration or the like by exciting the gas using laser light to produce fluorescence and analyzing the state (the wavelength, the intensity of the wavelength, and so forth) of the fluorescence (for example, refer to Japanese Patent Laid-Open No. Hei 8 (1996)-75651 (hereinafter, referred to as Patent Document 1)). The outline of the theory of operation of this method is now described with reference to FIG. 7. The object gas of analysis or plasma of the gas is irradiated with laser light for excitation in order to allow an atom in a lower level 1, which is a normal energy level (ground level), among atoms constituting the gas or plasma to absorb the energy of the laser light, by which the atom is excited to an upper level as indicated by an arrow Ya. In this instance, the wavelength of the laser light is set to the same level as the wavelength corresponding to the energy difference between the lower level 1 and the upper level (the wavelength is proportional to a reciprocal of the energy difference), that is the wavelength (resonance wavelength) corresponding to the energy to be absorbed when the atom is excited from the lower level 1 to the upper level. Therefore, the energy absorbed by the atom during the excitation is equal to energy of a photon of the laser light for excitation.
The gas or plasma excited to the upper level as described above is thereafter deexcited to the lower level 1 or 2 as indicated by an arrow Yb or Yc and produces fluorescence A or B during the deexcitation. In this instance, it is assumed that the lower level 2 exists between the lower level 1 and the upper level. In the analytical method as described in the Patent Document 1, the fluorescence A or B is picked up and the state (the intensity or the like) thereof is analyzed.
In addition, as an analytical method applicable only if the object gas of analysis has a molecular structure, the Raman scattering method (the spontaneous Raman scattering or stimulated Raman scattering (CARS)) is also conventionally known (for example, refer to Japanese Patent Laid-Open No. Hei 9 (1997)-292341 (hereinafter, referred to as Patent Document 2)). The Raman scattering method includes rotationally or vibrationally exciting molecules using laser light and analyzing the state of light scattered from the molecules (scattered light) during the excitation.
Furthermore, there is already known a method of detecting visualized hydrogen by visualizing hydrogen using a hydrogen visualization agent such as iodate ion solution to react iodine, which has been generated by reacting the hydrogen visualization agent with the hydrogen, with starch (for example, refer to Japanese Patent Laid-Open No. 2001-147225 (hereinafter, referred to as Patent Document 3)).
On the other hand, for example, regarding a fuel cell stack, it has been required to detect a gas leak before shipment or the like of the fuel cell stack since the gas leak from its body part or a pipe connected thereto leads to a reduction in generating efficiency or other troubles.
In the conventional fluorescence analysis method as disclosed in the Patent Document 1, the wavelength of the laser light emitted for irradiation needs to be the same as the wavelength corresponding to the inter-level energy difference of the atom or plasma as described above. Therefore, the wavelength of the laser light needs to be varied according to the type of gas to be detected (gas for producing fluorescence). Accordingly, the conventional fluorescence analysis method has a problem that it requires a device for varying the wavelength of the laser light when analyzing various types of gas using the conventional fluorescence analysis method, which leads to a complicated device configuration and to an increase in device cost. Furthermore, where gas is changed to plasma (dissociated electrolytically) to increase the intensity of the produced fluorescence, it is necessary to use two laser devices, which leads to a more expensive and complicated device.
Moreover, in the conventional fluorescence analysis method, some types of gas (for example, hydrogen and helium) cannot be detected as a matter of fact for the reason described below. More specifically, hydrogen or helium has a large inter-level energy difference (10 eV or higher) and therefore the absorption wavelength for its excitation exists in an extreme vacuum ultraviolet region of 120 nm or less. Under present circumstances, there is no laser device or optical component that can easily generate laser light having a wavelength in the extreme vacuum ultraviolet region. Furthermore, the light in the extreme vacuum ultraviolet region is immediately absorbed and attenuated in the air and therefore it is very difficult to irradiate a desired position with that kind of light.
In addition, since hydrogen and helium are useful as gas for use in leak inspection of a vacuum chamber or a fuel cell (fuel cell using hydrogen as fuel), it is required to develop a technology for optically detecting the hydrogen or helium without using the extreme vacuum ultraviolet light.
Moreover, the Raman scattering method as described in the Patent Document 2 is applicable only to some types of gas such as, for example, hydrogen molecules and nitrogen molecules, which are molecules that can be vibrationally or rotationally excited by irradiation with laser light and whose polarizability varies remarkably during the excitation. Accordingly, monoatomic gas (rare gas) such as helium or argon and a hydrogen atom cannot be detected by the Raman scattering method, by which the object gas of analysis is limited disadvantageously. Furthermore, in the spontaneous Raman scattering of the Raman scattering method, it is difficult to detect a very small amount of gas in the air. Still further, in the stimulated Raman scattering of the Raman scattering method, two different kinds of laser light are needed, which leads to a large-sized or complicated device configuration and to an expensive device disadvantageously.
Regarding the method using the hydrogen visualization agent as described in the Patent Document 3, it is difficult to supply the hydrogen visualization agent, which is liquid, only to a region within a specific small area of a fuel cell stack, for example, when it is attempted to detect a leakage of the hydrogen while supplying the hydrogen to the fuel cell stack. Therefore, it is difficult to identify the region of occurrence of a gas leak accurately using the method. Moreover, it has a problem of requiring a work for removing hydrogen visualization agent adhered to the surface of the fuel cell stack after the inspection.
In addition, it is also possible to soak the fuel cell stack in water and to supply the fuel cell stack with gas in this condition and to observe bubbles generated in the water in order to inspect the fuel cell stack for a gas leak. This method, however, requires a work for drying the fuel cell stack after the inspection.
In view of the above background, the present invention has been provided. Therefore, it is an object of the present invention to provide a laser analytical instrument and a laser analytical method capable of observing various types of gas through an inexpensive and simple arrangement independently of the type of gas employed. Furthermore, it is another object of the present invention to provide a gas leak inspection instrument capable of easily detecting a gas leak of a fuel cell stack or the like by using laser light.